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Open Roads Forum  >  Fifth-Wheels

 > Ford super duty 3.31 rear end. Will it be a big mistake. ?

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fowlthing

Roger, AR

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Posted: 01/21/21 04:48pm Link  |  Quote  |  Print  |  Notify Moderator

Pulling a 37 foot Cedar creek with a f350. Diesel. Would the 3.31 rear end work or should I get the 3.55?


2017 Ford 350 SRW
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may switch to a Ram

cummins2014

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Posted: 01/21/21 04:57pm Link  |  Quote  |  Print  |  Notify Moderator

fowlthing wrote:

Pulling a 37 foot Cedar creek with a f350. Diesel. Would the 3.31 rear end work or should I get the 3.55?


Towed a 38' Montana with 3:42's with my 2014 6.7 Cummins , not a problem . I know the Ford is a different beast , but those 3.55 might do just a bit better . I personally wouldn't want any lower gears with the Ram then what I got. But the new Fords also have the 10 speed trans, so there may be some that will have a better answer, possibly those 3:31's are all you would need for what a 15-16 GVWR fifth wheel ?

mlburst

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Posted: 01/21/21 05:35pm Link  |  Quote  |  Print  |  Notify Moderator

I've been towing with the 3.31 rear end since 2012 - coupled with the six speed tranny it's perfect - flat land or mountains makes no difference.


Mike & Joanna
Myrtle Beach SC/Port Charlotte FL
"Any-timers" since Dec 2010
2010 Heartland Sundance 2900MK
2012 Ford F250 Lariat King Ranch CC 6.7L Diesel
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rhagfo

Portland, OR

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Posted: 01/21/21 05:49pm Link  |  Quote  |  Print  |  Notify Moderator

mlburst wrote:

I've been towing with the 3.31 rear end since 2012 - coupled with the six speed tranny it's perfect - flat land or mountains makes no difference.
______________________________
Mike & Joanna
Myrtle Beach SC/Port Charlotte FL
"Any-timers" since Dec 2010
2010 Heartland Sundance 2900MK
2012 Ford F250 Lariat King Ranch CC 6.7L Diesel
Heartland Owners Club #1452
Escapees #99187


Well not quite apples to apples, your Heartland 2900 weighs in dry at 9,170#, and has a payload of 4,760#, unless you full time likely only use 2,000# of that so towing about 11,170#. That is a big difference to the OP's likely 16,000# 5er. I would go with at least 3.55's or 3.73's.


Russ & Paula the Beagle Belle.
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JTrac

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Posted: 01/21/21 06:04pm Link  |  Quote  |  Print  |  Notify Moderator

I've had mine with the 3.31 for a year now. I have not regretted it for one single minute. We have towed the new Pinnacle, which weighs 14,700 loaded for a trip, for about 5K miles, to include a trip to the Colorado Rockies last fall. It tows as well or better than any combo we have ever had and that goes back for 40 years for us. The 10 speed is amazing in my books. It is so smooth and when I do get a downshift while towing I rarely even notice it. Usually it just drops to 8th gear. It can hold 10th gear surprisingly well most of the time.

I wanted the 3.31 because we use our truck for road trips as much as we do towing. Here is a picture of my trip meter on a recent trip to the sil's house north of Chicago. Either rear end will work and work well but I've certainly had no issues with the 3.31.

Couldn't get the picture to work but the mileage was 21.8 for 733 miles.


JimT
2020 Jayco Pinnacle 32RLTS, 2020 Ford F350, Platinum, 6.7 diesel, 4X4, CCLB, SRW, 12,400 GVWR

Lynnmor

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Posted: 01/21/21 06:06pm Link  |  Quote  |  Print  |  Notify Moderator

Of course the 3.55 will do better, all other things being equal. When doing your homework, check that the tire sizes are the same, earlier 6.7s used the 3.55 gears with the 20" wheels and the 3.31 with the 18" wheels, both had the same engine speed. There were a few exceptions such as snow plow preparation.





Lantley

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Posted: 01/21/21 06:16pm Link  |  Quote  |  Print  |  Notify Moderator

If buying the truck for towing get the 3.55. Why not? All things equal the 3.55 will perform better towing. The only advantage I see for the 3.31 is MPG's.
A F-350 will never be a commuter car it will always be a fuel hog regardless of the gearing.


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4x4ord

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Posted: 01/21/21 08:47pm Link  |  Quote  |  Print  |  Notify Moderator

I've got a 2017 F350 with 3.55 gears and 20" wheels that I tow a 16k lb rv with. My 2021 will be 3.31 gears. The 10 speed has a pretty low ratio reverse which is going to be a downer even with 3.31 gears.

* This post was edited 01/21/21 08:56pm by 4x4ord *


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Nv Guy

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Posted: 01/21/21 10:32pm Link  |  Quote  |  Print  |  Notify Moderator

The Ford towing guide doesn't draw much difference between 3:55 / 3:31.
Between the 10 speed and the torque the Diesel delivers it will be fine.
I wouldn't classify an F350 as a fuel hog, the '77 Dodge I had was a fuel hog -8 MPG empty.





valhalla360

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Posted: 01/22/21 06:57am Link  |  Quote  |  Print  |  Notify Moderator

This has been beat to death on recent threads.

With the 8/10speed transmissions, the rear end ratio doesn't make much difference.
- With 1 key exception, if you would be over the max tow limit with the lower numerical ratio.

What the engine will see is it needs to provide roughly the same RPM/Torque for the same speed regardless of rear end. The transmission will select a gear that makes that happen.

You might run in 7th instead of 8th but who cares if the motor is running at the same RPM/Torque output?

In the old day's of 3 or 4 speed transmissions, it made a big difference because there was so much room between gears that if you dropped down a gear, you could miss the engines RPM sweet spot (peak torque/HP) and it would result in running at excessive RPM. Using a higher (numerical) rear end could compensate by adding to the torque allowing you to stay in a higher gear. With 8/10 gears and motors designed with flat torque curves (not to mention massive torque compared to 20yrs ago), the truck can always keep the motor running darn close to ideal RPM for conditions.


Tammy & Mike
Ford F250 V10
2021 Gray Wolf
Gemini Catamaran 34'
Full Time spliting time between boat and RV


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